Evidence is accumulating that training with mental imagery of strong muscle contractions enhances descending command (from the brain) and strengthens muscle output. This finding has substantial application in medical rehabilitation because although it is unsafe or difficult for many cognitively healthy patients and frail older adults who suffer muscle weakness to undergo conventional strength training, it is possible for them to use their mind to strengthen their muscles. Two major obstacles to the implementation of mental training of this type are the inability of investigators to monitor the mental processes of the subjects directly and the difficulty of many subjects, especially those with reduced cognitive ability (e.g., old people), to mentally contract a muscle strongly without any feedback about relevant brain activity. It has recently been shown that power of scalp EEG frequencies is linearly correlated to the intensity of muscle contraction. Furthermore, the EEG oscillation power increases proportionally with intensity of intended (mental) muscle contractions. These findings indicate that the EEG power signal reflects brain activities controlling muscle force; this offers the possibility of using the EEG frequency power as an objective parameter for monitoring quality of the mental training and maximizing the outcome result. Our overall hypothesis is that in given subjects with muscle morphology and coordination unchanged, the amount of strength is directly related to the magnitude of brain signal driving the muscle; mental training with the cortical signal feedback can more effectively enhance the brain signal and increase muscle strength. Aim 1 will determine whether mental training by monitoring the power of EEG frequency is superior in allowing gain in muscle strength to mental training without such feedback in older adults. Aim 2 will examine potential neural mechanisms that mediate mental training-induced strength improvements. It is hypothesized that the mental training group with the EEG feedback will exhibit greater cortical signal change and gain more strength than the group with no such EEG feedback; the amount of neural adaptation will correlate significantly with strength increases.